Vitamin D, phagocyte differentiation and immune function.

Abstract

The data reviewed in this paper supports the hypothesis that 1,25-(OH)2D3 influences the differentiation and function of mononuclear phagocytes and lymphocytes. A caveat is the concentrations of 1,25-(OH)2D3 required in vitro for expression of these effects. The relevance of these concentrations to the actions of 1,25-(OH)2D3 in vivo is speculative because the experimental conditions involve the presence of serum (and vitamin D-binding protein) which undoubtedly alters the concentration of 1,25-(OH)2D3 entering the incubated cells. Serum is a critical reagent in most cell culture systems and it is not yet possible to separate the effects of serum from the effect of the 1,25-(OH)2D3 contained in serum. Our current hypothesis is that 1,25-(OH)2D3 in normal serum plays a permissive role in the functions of normal mononuclear phagocytes and lymphocytes, which are exaggerated at higher concentrations of 1,25-(OH)2D3. These data also suggest that skeletal homeostasis has immunologic dimensions. Lymphokines appear to be important in the formation of osteoclast-like cells. Lymphokines are also believed to be involved in some forms of hypercalcemia associated with malignancy such as multiple myeloma [80] and the adult T-cell lymphoma syndrome [81]. It is clear that the interactions between vitamin D, mononuclear phagocytes, lymphocytes and their products, and skeletal homeostasis have introduced new perspectives and dimensions to areas previously perceived as being unrelated or, at least, distantly linked.